Question

In order to save energy, you decide to boil water for your tea by shaking it in a thermos flask. Suppose you use $\frac{1}{2}kg$ of tap water at ${20}^{o}C$ and water falls $0.1m$ in each shake and you make $30$ shake each minute. Neglecting any loss of heat by the flask calculate how long must you shake the flask so that water begins to boil? Given that $g=10m/s^2$ and specific heat of water is $4200J/kg$?

Answer 1

Change 10 potential energy

$=$ energy required to boil water

$\therefore 30\times mgh=mc\Delta T$

$\therefore t\times 30\times 10\times 0.1=4200(100-20)$

where $t\to$ time in minutes.

$\therefore t=4200\times 80$

$\therefore t=1120min\to$ ans.